The US has refused to accept hardline president Mahmoud Ahmadinejad's claim of a landslide re-election victory in Iran and said it was looking into allegations of election fraud.
Any hopes by the Obama administration of gaining a result similar to Lebanon's recent election, won by a Western-backed moderate coalition, appeared to be in jeopardy.
"We are monitoring the situation as it unfolds in Iran, but we, like the rest of the world, are waiting and watching to see what the Iranian people decide," US secretary of state Hillary Rodham Clinton said at a news conference with Canada's foreign affairs minister, Lawrence Cannon.
Minutes after Mrs Clinton spoke, the White House released a two-sentence statement praising "the vigorous debate and enthusiasm that this election generated, particularly among young Iranians," but expressing concern about "reports of irregularities."
US vice president Joe Biden today cast doubt on Mr Ahmadinejad's re-election, saying Tehran's actions against dissent suggest the results may not be clear-cut.
"It sure looks like the way they're suppressing speech, the way they're suppressing crowds, the way in which people are being treated, that there's some real doubt," Mr Biden told NBC's "Meet the Press" when asked if Mr Ahmadinejad had won the vote.
"I have doubts but we're going to withhold comment until we have a thorough review of the whole process and (see) how they react in the aftermath," he said.
Official election results gave Mr Ahmadinejad nearly 63 per cent of the vote and only 34 per cent for Mr Mousavi.
"That's what they're announcing. We have to accept that for the time being. But there's an awful lot of question about how this election was run," Mr Biden said.
But he cautioned: "We don't have enough facts to make a firm judgment."
In expressing his doubts, Mr Biden pointed specifically to the vote count among Iran's cities, which he said account for 70 per cent of the country's balloting but suggested that urban voters were not expected to turn out heavily for Mr Ahmadinejad.
"The idea he'd get 68 or whatever per cent of the vote in a circumstance like that seems unlikely," the vice president said.
Despite the challenge from reformist Mir Hossein Mousavi to incumbent Mr Ahmadinejad, many officials and experts thought a Mousavi victory would result in only incremental shifts toward the US.
Because real power in Tehran is still wielded by religious leader Ayatollah Ali Khamenei, some say an Ahmadinejad re-election may make it easier to build an international consensus against Iran.
Administration officials remained silent out of concern that any comments might influence the results. But they were privately hoping for a victory by the more moderate Mr Mousavi.
President Barack Obama's previous overtures include his recent address in Cairo to the Muslim world as well as, earlier, a televised New Year's address to the Iranian people and a series of diplomatic contacts. Officials say Obama's attempts to reach out have gone largely unanswered.
Neither Ms Clinton nor the White House mentioned Mr Ahmadinejad or his chief rival Mr Mousavi, by name, or acknowledged the incumbent's victory declaration.
Mr Mousavi, a former prime minister who has become the hero of a youth-driven movement seeking greater liberties and a gentler face for Iran abroad, rejected the results and accused authorities of rigging Friday's vote.
In brief remarks in Canada, Mrs Clinton cited "the enthusiasm and the very vigorous debate and dialogue" in the run-up to the vote. "We obviously hope that the outcome reflects the genuine will and desire of the Iranian people," she said.
The election focused on what the office of the Iranian president can influence: boosting Iran's sinking economy, pressing for greater media and political freedoms, and being Iran's main envoy to the world.
Iran does not allow international election monitors. During the 2005 election, when Mr Ahmadinejad won the presidency, there were some allegations of vote rigging from losers, but the claims were never investigated.
Agencies